The present invention is related to manually operated muscle building exercise apparatus, specifically to apparatus that include a resilient elastic cable to provide resistance to motion. Such elastic cable exercise devices are particularly useful in that, as contrasted with barbells and dumbbells, a substantial resisting force can be generated by a relatively lightweight, portable device.
A principal drawback to elastic cable exercise devices, however, is the difficulty encountered in attempting to adjust the restoring force. Typically, the restoring force is adjusted by increasing or decreasing the free length of the elastic cable. For example, U.S. Pat. No. 4,779,867 to Hinds discloses an elastic cable exercise device comprising a cable having stirrups at each end and a bar that engages the central portion of the cable to provide a handle for the user to pull against the cable. Hinds discloses that the effective free length of the cable is adjusted by wrapping the cable about the ends of the bar. As noted in Hinds, however, wrapping the cable about the bar to shorten the effective length has an untoward side effect in that, in use, the cable exerts a torque on the bar and/or can slip off the end of the bar. Hinds therefore discloses an improvement comprising a pair of lugs at each end of the bar designed to retain the cable to prevent the wrapped cable from slipping over the ends of the bar during use and to prevent the cable from exerting a torque on the bar. Hinds does not, however, address the inherent limitations in the range of restoring force adjustments that can be made in such exercise devices employing a single cable.
Single cable elastic cable exercise devices suffer from an inherent limitation in the range of restoring force adjustment that can be made, because the only practical method for making adjustments is to shorten or lengthen the effective length of the cable. Elastic cables are similar to springs in that they exert a force that is proportional to displacement. Although elastic bands do not behave linearly, as do metallic springs, elastic bands can nevertheless be characterized as having an effective spring rate. Since, like a spring, an elastic cable exerts a force that is a function of displacement, elastic cable exercise devices do not exert a constant restoring force as do ordinary weight sets. Therefore, in order to simulate as closely as possible the constant force exerted by an ordinary weight set, elastic cable exercise devices are typically operated in such a way that the tensioned length of the cable changes by the minimum percentage possible over the full range of the exercise. A constant force is most nearly simulated using the a long cable (of low spring rate) stretched initially to provide the desired preload, which is then exercised over a short stroke. Obviously, this arrangement is not feasible in many instances.
Shortening or lengthening the effective length of the elastic cable to adjust the preload can accomplish only a very limited variation in the restoring force because the preloaded cable must still have sufficient reserve stretch to extend through the full range of motion of the particular exercise. Beyond a certain point, typically about 300% or so, a latex cable exhibits a rapid increase in its effective spring rate. Accordingly, if a large reduction in the free length of the cable is attempted to achieve a substantial increase in preload, the result will be a cable that cannot be stretched through the full range of motion necessary to perform the exercise. Thus, to accommodate a full range of potential users, manufacturers of single cable elastic cable exercise devices must provide a selection of interchangeable cables of different effective spring rates, with the concomitant increase in cost, and decrease in ease of use.